The present invention relates generally to current switches, and more particularly to half current switches with excellent noise rejection and fast switching.
Present half current switch configurations provide proportionate IR output voltage drop variations in response to variations (noise) in the HIGH input level at their input transistors. Such IR drop voltage variations, if large enough, can result in erroneous switching operations in the system.
One solution to this noise switching problem is to connect a diode or the collector and emitter of an NPN transistor in parallel across the load resistor connected to the input transistor. Such a connection effectively holds the voltage across the load resistor to a prescribed value when it is drawing current. However, the extra current drawn by the clamping diode or NPN transistor increases the power consumption of the circuit and causes an increased emitter-base capacitance in the input transistor, resulting in a slower switching action. Note that the input capacitance of a transistor increases proportionately with increases in current through the transistor's emitter.
The invention as claimed is intended to remedy the above-described power dissipation and switching speed draw-backs to the half current switch. Additionally, the invention is designed to permit the use of a speed-up capacitance therein while suppressing circuit oscillation.